Fetal venous circulation--an update on hemodynamics.

The refinements of modern ultrasound techniques permit a renewed examination of old concepts of fetal circulation. The concept of preferential streaming of umbilical blood through the foramen ovale is verified by animal experiments, and ultrasound studies have confirmed that a similar mechanism operates in human fetuses. However, the normalized umbilical flow appears to be less in the human than in fetal sheep, and decreases with advancing gestational age (115 ml min-1 kg-1 at 20 and 64 mL min-1 kg-1 at 40 weeks). Compared to the 50% shunting of umbilical blood through the ductus venosus found in animal experiments, the degree of shunting in the human fetus under physiological conditions is considerably less, 30% at 20 weeks, which decreases to 18% at 32 weeks, suggesting a higher priority of the fetal liver than previously realized. Augmented pulsatility in the precordial veins, ductus venosus, and umbilical vein is an important clinical sign that is poorly understood. Recent fluid dynamic studies show that, apart from the pressure generated in the atria, it is the stiffness of the vessel wall, compliance, and notably, impedance which modify these waves. Particularly the substantial shift in impedance at the ductus venosus-umbilical vein junction causes wave reflection and reduced transmission of waves, the result being diminished or absent pulsation in the umbilical vein.